There are many occasions when fluid must be forceably pumped through a flow line which leads to a high-pressure point of use. For example, when acidizing boreholes, fluid must be pumped into the borehole against a considerable amount of back pressure, and provision must be made by which no back-flow can occur until the well treatment has been completed. Moreover, in drilling boreholes, it is necessary to pump drilling mud through the drill string and downhole where the lower borehole pressures may occasionally exceed the hydrostatic head of the well; and accordingly, provisions must be made by which no back-flow can occur from the drill string back to the mud pumps.
In carrying out the above-described operation, the component parts of a check valve assembly are subjected to corrosion and abrasion; and therefore, the valve must be built and designed in such a manner that it can be easily overhauled from time to time. Otherwise, the above-mentioned deleterious effects will render the valve inoperative in a very short while, and the expense of operation becomes excessive. Therefore, it is desirable to have a drop check valve made available which is highly efficient in operation, and which can easily be overhauled with a minimum of cost and effort.